I would definately be concerned about fat and calories as well. I say, ditch the mayo altogether. Even the low-fat and low-cal types have very little else to offer except empty calories. Try mustard instead or even a very light dressing mixed with herbs. Try grilling your sandwiches. This tends to make them a little moister so you don't need to add a dressing to it or use more juicy veggies like tomatos.
Some days I just need something on my sammie. Then, I use Krafts fat free ranch or fat-free zesty italian. They have no fat and 15 cal per serving (but you probably won't need a whole serving on your sammie!)
When I first started I was using lots of Fat-Free Miracle whip--I used it in all kinds of stuff--egg salad, chicken salad, cole slaw or whatever..I still use it from time to time but have found my need for it has really slowed..
I also use Hellmann's or Kraft...yes, it does contain sugar...but, I've never been able to find a brand with no sugar. As far as fat...on Atkins, using full fat mayo is encouraged
you could also try marie's ceasar dressing.....it appears to have alot less sugar than hellmans. This is the pourable one. The jar type I believe does not have any sugar in it.